This application relates generally to the field of information storage and more particularly to a method for increasing the reliability of data storage at high areal densities.
The need for more efficient data storage devices has become critical with the staggering pace of advances in computer technology. The most common data storage device used today is the disc drive. Most disc drives are composed of one or more magnetic media discs attached to a spindle. A spindle motor rotates the spindle and discs at a constant high speed. An actuator assembly adjacent to the disc(s) has actuator arms extending over the discs, each with one or more flexures extending from each actuator arm. A read/write head is mounted at the distal end of each of the flexures. The read/write head includes an air bearing slider enabling the head to fly in close proximity above the corresponding surface of the associated disc. Information is stored on and retrieved from the magnetic media discs via the read/write head.
Currently, the disc drive industry utilizes longitudinal recording technology. With longitudinal recording, a bit of information is stored by orienting the magnetization direction of each domain on the disc surface lengthwise in the direction of rotation of the disc. A bit may be made up or one or more domains. A domain may consist of one or more magnetic grains, where a grain may consist of one or more atoms. Each disc is divided radially into a finite number of concentric tracks to organize the stored bits. Each track is a certain width and is separated from the adjacent tracks by a xe2x80x9cblank spacexe2x80x9d. This blank space prevents information stored in one track from overlapping the information stored in an adjacent track. The number of tracks located on each disc surface is known as the track density. Each track is subdivided into sections, called segments. Bits are written to and read from these segments by the read/write head. The linear density of bits stored within each segment is called the bit density.
The product of track density and bit density is known as areal density. The recent trend being followed by disc drive manufacturers is to increase the recording media""s areal density so that the amount of data stored can be increased without increasing the physical size or the number of discs used in a drive. For example, the areal density of early disc drives was less that 1 Gbits/sq. inch, whereas today, disc drives with areal densities greater than 40 Gbits/sq. inch are being tested. Manufacturers increase areal density by increasing both track density and bit density. Narrowing track width and/or narrowing the width of the blank spaces between tracks increase track density. Bit density is usually increased by increasing the recording speed in order to record higher frequency bits; a higher frequency bit takes up less space on the disc surface.
Packing more information onto a given size magnetic media has certain drawbacks. As mentioned above, the individual magnetic bits located on the recording media become smaller as areal density increases. Furthermore as the frequency of a magnetic bit stored in a magnetic domain increases, the magnetic bit size decreases. However as the frequency of the bit rises, the tendency of the bit orientation to decay, or disorient, increases exponentially, thereby increasing the risk of data loss. (For example, a bit with the frequency of 50.8 MHz may decay over a period of 1000 years; whereas, a bit with only twice that frequency, 101.6 MHz, may decay over a period of 10 years.) At a certain point, magnetic bits become so small that they are unstable at room temperature and spontaneously decay, thereby making data storage impossible. This phenomenon is known as the superparamagnetic effect. Because of the superparamagnetic effect, the amount of data that can physically be stored on a disc surface is limited. It has been predicted that about 100 Gbits/sq. inch is the highest density of data that can be achieved on a magnetic disc using the longitudinal recording method. Furthermore, with the current rate of technological advances, it is predicted that the superparamagnetic limit will be reached within the next two years.
Disc drive manufacturers are investigating non-longitudinal recording techniques such as vertical recording, holographic recording, and special signaling or orientating of the storage media among others in an attempt to overcome the 100 Gbits/sq. inch limit caused by the superparamagnetic effect. However, longitudinal recording offers cost, manufacturing, and technological advantages that the disc drive manufacturers wish to exploit.
Accordingly, there is a need for means or method of exploiting the benefits of longitudinal recording while avoiding or compensating for the superparamagnetic limit.
Against this backdrop the present invention has been developed to increase the amount of data that can reliably be stored on magnetic media. The present invention offers a means to compensate for the superparamagnetic effect and increase the amount of data that can be reliably stored on a recording media by monitoring the decay of the magnetic bits and refreshing the magnetic bits when the amount of decay has passed a pre-established threshold. The present invention can be used for any type of magnetic media storage system such as disc drive and magnetic tape drives among others. However, a disc drive has been used for illustrating the invention.
According to the present invention, a high frequency reference signal and a low frequency reference signal may be either written onto the magnetic media during the disc drive manufacturing process or during normal operation of the drive. The low frequency reference signal corresponds to larger magnetic bits on the storage media surface, whereas the high frequency reference signal corresponds to smaller magnetic bits on the media surface. In the case of a disc drive, these signals can be written to a dedicated system track (called a xe2x80x9csuper trackxe2x80x9d) or to a dedicated system track sector (called a xe2x80x9csuper sectorxe2x80x9d). The reference signals can contain important parametric information such as amplitude, bit error rate, signal to noise ratio, and spectrum information (FFT of the signal).
The low frequency reference signal, because it decays at an exponentially slower rate than the high frequency reference signal, can be used as a baseline to determine the amount of decay in the high frequency signal. Any change in the difference between the two signals"" parametric information actually reflects the level of signal degradation that has occurred in the high frequency reference signal. In other words, the low frequency reference signal""s parametric information remains constant (for all practical purposes) when compared to the high frequency reference signal""s parametric information. Therefore, if the difference between the two signals"" parametric information increases, the increase can be attributed to the high frequency reference signal""s decay. Furthermore because the high frequency reference signal corresponds to the higher density and smaller magnetic bit size on the media, any degradation of the high frequency reference signal can be used as an indication of decay in the media""s data domains.
When the present invention is applied to a disc drive, the disc drive""s read/write head is used to read the high and low frequency reference signals; an average of many read operations is preferably used to determine the difference (xe2x80x9cxcex94xe2x80x9d) of the parametric information between the high frequency reference signal and the low frequency reference signal. The difference determined after first writing the signals on the magnetic media (xe2x80x9cxcex94originalxe2x80x9d) can be recorded on the disk media and/or in system memory as a reference baseline.
Each time the disc drive is subsequently activated, after the signals have first been written on the magnetic media, it can perform a self-diagnostic test. The same real time parametric information, as measured to determine xcex94original, is measured and recorded. This real time difference measurement (xe2x80x9cxcex94nxe2x80x9d) is then compared with xcex94original. The difference between xcex94n and xcex94original (called xe2x80x9cxcex4xe2x80x9d) is compared to a predefined threshold limit. If xcex4 exceeds the threshold limit, the user can be warned that a backup and/or refresh procedures should be completed. Storage devices such as CD-ROM""s, storage attached network (xe2x80x9cSANxe2x80x9d), network attached storage (xe2x80x9cNASxe2x80x9d), among others, can be utilized for this back up. Alternatively, the data on the disc can be automatically backed up and/or refreshed without user input. The high frequency and low frequency reference signals can also be refreshed in order to establish a new baseline for the subsequent delta calculation (i.e., a new xcex94original).
In accordance with other aspects, the present invention relates to an apparatus for increasing the amount of data reliably stored on magnetic media. The apparatus includes a controller for writing a high frequency and a low frequency reference signal onto the magnetic media, reading the signals from the magnetic media, determining the difference between the signals, and comparing the difference to a threshold. If the difference exceeds the threshold, the controller notifies the user that the data stored on the magnetic media needs to be refreshed or backed up. Storage devices such as CD-ROM""s, storage attached network (xe2x80x9cSANxe2x80x9d), network attached storage (xe2x80x9cNASxe2x80x9d), among others, can also be utilized for this back up. Alternatively, the apparatus can automatically refresh and/or back up the data stored on the magnetic media without user input.
These and various other features as well as additional advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.